Method of fabricating isolated semiconductor devices in epi-less substrate

ABSTRACT

An structure for electrically isolating a semiconductor device is formed by implanting dopant into a semiconductor substrate that does not include an epitaxial layer. Following the implant the structure is exposed to a very limited thermal budget so that dopant does not diffuse significantly. As a result, the dimensions of the isolation structure are limited and defined, thereby allowing a higher packing density than obtainable using conventional processes which include the growth of an epitaxial layer and diffusion of the dopants. In one group of embodiments, the isolation structure includes a deep layer and a sidewall and which together form a cup-shaped structure surrounding an enclosed region in which the isolated semiconductor device may be formed. The sidewalls may be formed by a series of pulsed implants at different energies, thereby creating a stack of overlapping implanted regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/067,248,filed Feb. 25, 2005, which is a continuation of application Ser. No.10,218,668, filed Aug. 14, 2002, now U.S. Pat. No. 6,900,091, each ofwhich is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to semiconductor device technology and inparticular to complementary metal-oxide-silicon (MOS) devices that areelectrically isolated from each other and from the substrate in whichthey are formed.

BACKGROUND OF THE INVENTION

In the development of complementary MOS (CMOS) devices, there has been acontinual effort to fit more devices into a given area of asemiconductor wafer. FIGS. 1-5 illustrate several stages of thatdevelopment.

FIG. 1A illustrates a standard CMOS structure that would normally beused in devices having a feature size of 1.2 μm or larger. CMOS 10includes a P-channel MOSFET 10 a and an N-channel MOSFET 10 b and isformed in a P substrate 11. Typically, many other NMOSFETs and PMOSFETswould be formed in P substrate 11. P-channel MOSFET 10 a is formed in anN-well 14, which is formed by a conventional implant and extendeddiffusion process. Thus N-well 14 is implanted into a relatively shallowdepth of substrate 11 and expands both vertically and horizontally whenexposed to a thermal process.

MOSFETs 10 a and 10 b are both lateral devices and include gates 12 a,12 b, respectively, that are separated from the substrate 11 by a gateoxide layer 16. PMOSFET 10 a includes a P+ source region 13 a, a P+drain region 13 b and an N+ contact region 13 c, which is used to makecontact with N-well 14. NMOSFET 10 b includes an N+ source region 14 aan N+ drain region 14 b and a P+ contact region 14 c, which is used tomake contact with P substrate 11, which is the body of NMOSFET 10 b, viaa metal contact 18. The channel regions under the gates 12 a, 12 b mayor may not contain a threshold adjustment implant.

Metal contact 18 is tied to the most negative voltage in the system,which is normally ground. Therefore, CMOS 10 cannot operate at voltagesvery far above ground. Moreover, NMOSFET 10 b shares a common bodyterminal with any other NMOSFET in CMOS 10, and any currents or noisethat are injected into substrate 11 are coupled to NMOSFET 10 b and anyother NMOSFETs in the device, since the NMOSFETs are not isolated,

In CMOS 10, the doping concentration of substrate 11 must be designed toset the electrical characteristics of NMOSFET 10 b. This limitation isameliorated in CMOS 20, shown in FIG. 1B, where NMOSFET 10 b is formedin a P-well 21, The main purpose of forming NMOSFET 10 b in P-well 21,however, is to control the breakdown and punchthrough characteristics ofNMOSFET 10 b. Since there is no PN junction between P substrate 11 andP-well 21, NMOSFET 10 b still shares the same body with any otherNMOSFET in CMOS 20 and any other substrate-connected device, since thebody terminal of NMOSFET 10 b is electrically common with P-substrate 11and since N+ regions 14 a and 14 b cannot be biased to large voltagesabove the potential of P-substrate 11.

FIG. 1C illustrates in general a process that can be used to fabricateCMOS 20. The process starts with the formation of a field oxide layer onP substrate 11. The substrate is masked, and N-well 14 is formed by animplant and diffusion of phosphorus. The substrate is again masked, andP-well 21 is formed by an implant and diffusion of boron.

Next, there are two variations of the process. In one, the active deviceareas are defined by a mask and the field oxide layer is etched from theactive device areas. In the other, the field oxide layer is stripped anda pad oxide layer is thermally grown. Field oxide regions are formed bya conventional LOCOS process, which includes defining the active deviceareas by patterning, a nitride layer and etching the nitride layer fromthe areas where field oxide is to be grown. A blanket phosphor implantis performed to form an N field deposition (NFD), and a mask is formedto define areas where boron will be implanted to form a P-fielddeposition (PFD). The field oxide regions are then formed in areas wherethe nitride layer has been removed, the nitride layer is stripped and asacrificial oxide layer is grown and stripped to repair crystal damageand remove any silicon nitride residues that might impair the propergrowth of the gate oxide.

A gate oxide layer is then deposited and a polysilicon layer isdeposited, doped, masked and etched to form the gates of the MOSFETs.The source and drain regions of PMOSFET 10 a are formed by masking thesubstrate and implanting boron, and the source and drain regions ofNMOSFET 10 b are formed by masking the substrate and implantingphosphorus and/or arsenic. An anneal is applied to drive in the boronand phosphorus/arsenic implants

A conventional interconnect formation process is then performed,including the deposition and etching of glass layers and the deposition(sputtering) of metal layers that contact the source, drain and bodyregions of PMOSFET 10 a and NMOSFET 10 b.

FIG. 2A shows a CMOS 30 that is produced using a more modern processthat is capable of fabricating devices with a smaller gate dimension.N-well 14 contains a PMOSFET 30 a and P-well 21 contains an NMOSFET 21.N-well 14 and P-well 21 are formed as complements of each other, i.e.the entire surface of substrate 11 is occupied by either an N-well 14 ora P-well 21. An oxide sidewall spacer 19 is formed on gates 12 a, 12 b.Oxide sidewall spacer inhibits the implanting of high-concentrationdopant into substrate 11 thereby forming lightly-doped P-regions 33 a,33 b adjacent the source and drain regions 13 a, 13 b in PMOSFET 30 aand lightly-doped N-regions adjacent the source and drain regions 14 a,14 b in NMOSFET 30 b. A silicide layer 32 is formed on top of gates 12a, 12 b. CMOS 30 is a non-isolated twin well CMOS that represents themajority of CMOS devices in the 0.25 μm to 1.2 μm range. Like NMOSFET 10b shown in FIG. 1B. NMOSFET 30 b shares a common body region with allother NMOSFETs in CMOS 30. Therefore, NMOSFET 30 b must be biased nearground and is sensitive to any noise that may appear in P substrate 11.

CMOS 40, shown in FIG. 2B, is similar to CMOS 30 but is formed in alightly-doped P-epitaxial (epi) layer 41 that is in turn grown on aheavily-doped P+ substrate 42. This is generally done to improve thelatch-up characteristics of the device by preventing lateral voltagedrops along the substrate. Heavily-doped P+ substrate 40 has a lowerresistivity that the P− substrate 11 shown in FIG. 2A. This is anindication of the problems that can occur in non-isolated devices thatshare a lightly-doped common body region. While the heavily-dopedsubstrate can reduce latch-up in a normal digital IC, it does not offersufficient protection against latch-up in power and high-current ICs.

“Epitaxial” refers to the growth of a single-crystal semiconductor filmon a single-crystal substrate of the same semiconductor. The word“epitaxial” is derived from the Greek meaning “arranged upon”. See, A.S. Grove, Physics and Technology of Semiconductor Devices, John Wiley &Sons (1967), pp 7-20.

FIG. 2C illustrates a process that can be used to fabricate CMOS devices30 and 40. In the case of CMOS 30 the process starts with P− substrate11; in the case of CMOS 40 the process starts with P+ substrate 42 andincludes growing P-epi layer 41 on top of P+ substrate 42. Thecomplementary well formation and LOCOS field oxide formation aresubstantially, the same as the processes described in FIG. 1C, The gateformation includes the formation of a metal layer by chemical vapordeposition on top of the polysilicon gate, followed by a silicidationprocess.

Following the gate formation, the substrate is masked and phosphorus isimplanted to form lightly-doped N-regions 34 a, 34 b. The mask isremoved and another mask is formed to define the lightly-doped P−regions 33 a, 33 b. BF₂ is implanted to form P− region 33 a, 33 b. Thesidewall oxide or glass is then deposited and etched to form sidewallspacers 38 a, 38 b, 39 a and 39 b.

The substrate is masked and arsenic is implanted to form N+ regions 14a, 14 b. The substrate is masked again and BF ₂ is implanted to formregions 13 a, 13 b. An anneal is performed to drive in the dopants.

The interconnect formation includes the deposition of two Al—Cu layerswith intervening dielectric layers. A rapid thermal anneal (RTA) isperformed, a glass layer is deposited, patterned and etched, and a Ti orTiN adhesion layer is deposited on the glass before the first Al—Culayer. Typically, the glass layer such as spin-on )lass or BPSG isplanarized by etchback or chemical-mechanical polishing (CMP) prior topatterning. The deposition of the second glass layer is followed by avia mask and etch, a tungsten deposition and etchback and the depositionof the second Al—Cu layer. The second glass layer, which may be achemical vapor deposition (CVD) layer with TEOS as a precursor or aspin-on glass (SOG) layer, should be formed at a low temperature toavoid melting the first metal layer. The tungsten plug is typically usedto planarize the via hole prior to the deposition of the second metallayer. The planarization is carried out by etchback or CNP.

FIG. 3A illustrates a substantially different approach to thefabrication of a CMOS device, using technology that evolved from thefabrication of bipolar devices. CMOS 50 includes an NMOSFET 50 a, formedin a P-well 56, and a PMOSFET 50 b, formed in an N-well 55. P-well 56and N-well 55 are formed in an N-epi layer 52 that is grown over a Psubstrate 51. NMOSFET 50 a includes an N+ source region 60 a and an N+drain region 60 b. Lightly-doped N regions 62 a, 62 b are formedadjacent to regions 60 a, 60 b, respectively. A gate is formed over agate oxide layer 65, and a silicide layer 59 is deposited on the gate.Contact to P-well 56 is made via a P+ region 61 c.

PMOSFET 50 b includes a P+ source region 61 b and a P+ drain region 61a. Lightly-doped P regions 63 a, 63 b are formed adjacent to regions 61a, 61 b, respectively. A gate is formed over gate oxide layer 65, andsilicide layer 59 is deposited on the gate. Contact to N-well 55 is madevia an N+ region 60 c.

Regions of N-epi layer 52 are isolated from each other by stacks of Pdiffusions, such as the stack containing a P buried layer 53 and P-well56, which are implanted at the top and bottom of N-epi layer 52 and thenheated so as to cause them to diffuse upward and downward until theymerge. The “thermal budget” (i.e., the product of temperature and time)that is necessary to cause P buried layer 53 and P-well 56 to diffuse inthis way is substantial and ends up setting many of the electricalcharacteristics of the arrangement, Moreover, P buried layer 53 andP-well 56 also diffuse in a lateral direction, and this limits thepacking density of the devices.

FIG. 3B illustrates a variation in which N buried layer 54 has beenreplaced by a hybrid N buried layer 71 in a CMOS device 70. N buriedlayer 71 is generally doped with phosphorus but contains a centralregion 72 that is doped with antimony. The phosphorus-doped portion of Nburied layer 71 has diffused upward to merge with N-well 55, eliminatingthe intervening segment of N-epi layer 52 that is shown in CMOS device50 of FIG. 3A. This provides a low-resistance path to N-well 55 andhelps to prevent latch-up resulting from lateral voltage drops in N-well55. Nonetheless, P-well 56 is still electrically tied to P-substrate 51,creating the limitations and problems described above.

FIG. 3C-3E are graphs of doping, concentration versus depth into thesubstrate at the cross sections indicated in FIGS. 3A and 3B. As thesegraphs suggest, the processes required to form these CMOS devices arehighly susceptible to variations in such parameters as epitaxialthickness diffusivity and temperature, and in addition they tend to bequite expensive, requiring long processing times and dedicatedhigh-temperature diffusion furnaces. The process shown, moreover,requires the P-type buried layer, the arsenic N-type buried layer andthe phosphorus N-type buried layer each to have its own dedicated mask,making the process even more expensive.

FIG. 4A is a schematic circuit diagram of CMOS devices 50 a and 50 b,shown in FIGS. 3A and 3B, respectively. Substrate 51 is shown as ground.PMOSFET 50 b is shown as isolated from ground by diode 97, whichrepresents the PN junction between P-substrate 51 and N buried layer 71.Diodes 95 and 96 represent the junctions between P+ source region 61 band P+ drain region 61 a, respectively, and N well 55. NMOSFET 50 a isshown as non-isolated. Diodes 92 and 93 represent the junctions betweenN+ drain region 60 b and N+ source region 60 a, respectively, and P well56.

FIG. 4B illustrates a PNP bipolar transistor that can also be formedfrom this process. P+ region could be the emitter. N well 55 and Nburied layer 71 could be the base, and P substrate 51 could be thecollector.

FIG. 5A shows a CMOS device 100 that contains three buried layers: an Nburied layer 103 (NBL2) of phosphorus underlying N well 104, a P buriedlayer 106 underlying P well 105, and an N buried layer 102 (NBL1) ofantimony, (or arsenic) that extends continuously under N well 104 and Pwell 105. PMOSFET 100 a and NMOSFET 100 b are similar to PMOSFET 50 aand NMOSFET 50 b shown in FIGS. 3A and 3B.

Extending N buried layer 102 under P well 105 has the effect ofisolating PMOSFET 100 a from the P-substrate 101. Thus all of theMOSFETS are isolated from the substrate. Adding N buried layer 102requires an additional mask, however, and the diffusion of N buriedlayer 102 during the long isolation diffusion adds still morevariability to the process. Therefore, it is necessary to overdesign allparameters including all updiffusion of buried layers, epi layer 114 mayhave to be grown to a thickness over 6 μm just to form 30V devices (thatideally less than 2 μm of silicon could support). In addition, thelateral diffusion of all the buried layers and the updiffusion of Nburied layer 102 that recurs during the isolation (well) drive-insfurther reduces the packing density that can be achieved.

FIG. 5B illustrates a possible process sequence for CMOS device 100. Theprocess starts with a P substrate on which a thick oxide layer isformed. A mask is formed for N buried layer 102 and antimony andphosphorus are implanted and allowed to diffuse by thermal processing.

Then a choice is made between a complementary buried layer process and amultiple buried layer process. In the multiple buried layer process,separate masks are used to define the locations of N buried layer 103and P buried layer 106, respectively. Each masking step is followed byan implant of either N-type dopant (phosphorus) or P-type dopant (boron)and after the implant the dopants are diffused by thermal processing. Inthe complementary buried layer process, a nitride layer is deposited andthen patterned and etched by using the CBL mask, followed by theimplantation of one of the two wells, which is subsequently oxidized.The nitride prevents the oxidation in the regions not receiving thefirst well implant, while the first well becomes covered by a thickoxide. The nitride is then stripped and the second well implant, thecomplement to the first, is executed. The thick oxide blocks the implantfrom the first well region. The second well is then diffused and all ofthe oxide is stripped. Hence, one mask defines complementary wells.

After the three buried layers have been formed, a P-epitaxial layer isgrown and the NMOS and PMOS devices are formed in the epitaxial layer asdescribed above. As will be apparent, this is a very complicated processinvolving numerous masking steps. It is possible, for example, to spend$150 just on the formation of the buried layers in a 6-inch wafer. If amistake is then made in the fabrication of the NMOSFETs or PMOSFETs,that cost is entirely lost. Moreover, the multiple diffusions that arenecessary create numerous possibilities for error, and even if thediffusions are carried out perfectly, the lateral diffusion of dopantthat is inherent in the process reduces the number of devices that canbe formed in a given area of substrate.

FIG. 5C shows a dopant profile taken at cross-section 5C-5C in FIG. 5A.This shows a region of the P-epitaxial layer between the N buried layer102 and the P well 105. In some cases N buried layer 102 merges with Pwell 105. This variability occurs mainly because P well 105 isreferenced to the top surface of the epi layer while N buried layer 102is referenced to the surface of P substrate 101. These variations canhave a significant effect on the electrical characteristics of a device,including junction breakdown, resistance, capacitance, speed andcurrent.

The schematic diagram of FIG. 5D shows the advantage of CMOS device 100.NMOSFET 100 a has a body that is tied to a separate terminal 110 a andcan be biased independently of the P substrate 101. Diode 127, whichrepresents the PN junction on between P well 105 and N buried layer 102,and diode 128, which represents the PN junction between N buried layer102 and P substrate 101, provide isolation for NMOSFET 100 a. Thecathodes of diodes 127 and 128 are N buried layer 102.

FIGS. 5A-5D demonstrate that to form an isolated structure a verycomplicated, costly process is required with numerous sources ofvariability and possible error. This process is suited primarily todevices having large feature sizes and large lateral spacing and can becarried out only in manufacturing plants capable of high temperatureoperations. This process is inconsistent with the modern CMOS processes,such as the process shown in FIG. 2A., which represents roughly 90% ofthe manufacturing capacity currently in existence. Thus there is a basicinconsistency between the processes required to produce isolated CMOSdevices and the manufacturing facilities available to produce suchdevices today. There is a definite need in the art of semiconductormanufacturing for a process that will overcome this problem.

SUMMARY OF THE INVENTION

In accordance with this invention, a high-energy implant is used tofabricate various structures for electrically isolating transistors andother devices from a semiconductor substrate and from each other.Alternatively, a series of implants at different energies can be used.In sharp contrast to the current practice the isolation structure anddevices are formed in a non-epitaxial semiconductor substrate. Thesubstrate is exposed to a very limited thermal budget and thus thespreading of the implants, both vertically and horizontally isrestricted.

In one groups of embodiments the isolation structure includes a deepisolating layer and sidewalls which extend upward from the buried layerto form a cup- or saucer-shaped structure of a first conductivity type,enclosing a region of a second conductivity type. The deep isolatinglayer can be formed by masking the surface of the substrate andimplanting dopant of the first conductivity type through an opening inthe mask to a predetermined depth below the surface of the substrate.The surface of the substrate may then be masked again, and dopant of thefirst conductivity type may be implanted through an opening, which maybe annular, to form the sidewalls of the isolation structure. Toincrease the height of the sidewalls, a series of implants may beperformed at different energies to create a vertical stack ofoverlapping doped regions.

The isolation region may be formed in a substrate of the secondconductivity type. The doping concentration of the region enclosed bythe isolation structure may be left unchanged or additional dopant ofthe second conductivity type may be added to form a well of the secondconductivity type. The well of second conductivity type may abut theisolation structure, or an intervening layer of the substrate with itsdoping concentration remaining unchanged may separate the well from theisolation structure. In still other embodiments, the well may, extendthrough the deep isolating layer and into the substrate beneath theburied layer. Two wells of the first and second conductivity types,respectively, may be formed in the region enclosed by the isolationstructure. The structure may contain two deep layers of first and secondconductivity type, respectively. The deep layer of the secondconductivity type may extend upward or downward, or both upward anddownward, from the deep layer of the first conductivity type. Thelateral dimension of the deep layer of the second conductivity type maybe smaller than the lateral dimension of the deep layer of the firstconductivity type.

The transistors or other devices may be formed in the region enclosed bythe isolation structure, or in the structure itself, or both.

In some embodiments the isolation on structure includes an implantedburied layer or well but no sidewalls.

The substrate is often biased at ground or the most negative on-chippotential, but this need not be the case.

Among the devices that can be isolated from the substrate using thetechniques of this invention, are N-channel and P-channel MOSFETs, PNPand NPN bipolar transistors, diodes, insulated gate bipolar transistors(IGBTs), resistors, junction field-effect transistors, photodiodes,detectors, or any other silicon devices.

Using the techniques at this invention avoids many of the problemsdescribed above. Dopants can be implanted with high precision to defineddepths in the substrate. By avoiding thermal diffusion processes—eitherdownward diffusions of dopants implanted through the top surface of anepitaxial layer or upward and downward diffusions of dopants introducedat the interface between all epitaxial layer and an underlyingsubstrate—both the horizontal separation between the devices and thehorizontal dimensions of the devices themselves can be reduced. Inaddition, the high costs associated with the growth of an epitaxiallayer can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate known CMOS structures.

FIG. 1C shows a process flow for forming the CMOS structure shown inFIG. 1B.

FIGS. 2A and 2B show CMOS devices produced using a more modern processthan the process shown in FIG. 1C.

FIG. 2C illustrates a process that can be used to fabricate CMOS devicesof FIGS. 2A and 2B.

FIGS. 3A and 3B illustrate an approach to the fabrication of a CMOSdevice using technology that evolved from the fabrication of bipolardevices.

FIGS. 3C-3E are graphs of doping concentration versus depth into thesubstrate at the cross sections indicated in FIGS. 3A and 3B.

FIG. 4A is a schematic circuit diagram of the CMOS devices shown inFIGS. 3A and 3B.

FIG. 4B is a schematic circuit diagram of a PNP bipolar transistor thatcan also be formed from the process used to make the devices of FIGS. 3Aand 3B.

FIG. 5A illustrates a CMOS device that contains three buried layers.

FIG. 5B shows a process for making the CMOS device of FIG. 5A

FIG. 5C shows a dopant profile of the (CMOS device shown in FIG. 5A.

FIG. 5D shows a schematic diagram of the CMOS device shown in FIG. 5A.

FIGS. 6A-6V illustrate a number of basic structures that can be formedusing the methods of this invention.

FIGS. 7A-7C illustrate devices in accordance with this inventioncontaining some of elements shown in FIGS. 6A-6V, including a fullyisolated CMOS device, an isolated NPN transistor, an N-channellightly-doped drain MOSFET (LDMOS), a lateral double-implanted P-channelLDMOS, a substrate PNP transistor, and a non-isolated NMOSFET.

FIGS. 8A-8H illustrate a process for forming an isolated P well inaccordance with the invention.

FIGS. 9A-9G are schematic diagrams that represent the devices shown inFIGS. 7A-7C.

FIGS. 10A-10F illustrate how the depth of the N deep isolating layer inthe substrate can be varied while still providing an isolationstructure.

FIGS. 11A-11G show a method of forming an isolation region using astair-step oxide.

FIGS. 12A-12F show a process of forming an isolation structure that usesa LOCOS technique.

FIGS. 12G-12O illustrate variations of the process shown in FIGS.12A-12F.

FIG. 13 illustrates several processes that can be used to form a fullyisolated twin well CMOS device.

FIGS. 14A-14H illustrate a “hybrid” process which combines theconventional diffusion of N and P wells with the subsequent implantingof a deep isolating N layer.

FIG. 15A is a graph showing the projected range R_(P)) of boron andphosphorus implants as a function of implant energy.

FIG. 15B is a graph of the straggle (ΔR_(P)) for similar implants ofboron and phosphorus.

FIG. 16A shows the vertical dimension between the bottom of a P+ regionand a deep isolating N layer in a P well and the vertical dimensionbetween the bottom of a P+ region and a deep isolating N layer in aregion of a P substrate.

FIG. 16B is a graph showing how the breakdown voltages of diodes varywith the vertical dimensions shown in FIG. 16A.

FIG. 16C shows the breakdown potential as a function of the implantenergy of the deep isolating N layer.

FIGS. 17A-17E illustrate how the range of the implant used to form thesidewall of the isolation region must be controlled to provide aneffective isolation region.

FIGS. 18A-18D illustrate how a series of implants can be employed toform a vertical sidewall of an isolation region.

FIGS. 19A-19D illustrate the steps of a process for fabricating anisolation region having a sidewall of the kind shown in FIGS. 18A-18D.

FIGS. 20A-20D show the steps of a similar process similar to that shownin FIGS. 19A-19D performed after field oxide regions have been grown onthe surface of the substrate.

FIG. 21A illustrates the horizontal diffusion of implants in a sidewallof an isolation region.

FIG. 21B illustrates an isolation structure formed by a deep isolatinglayer and an oxide-filled trench.

FIGS. 21C and 21D illustrate an isolation structure formed by implantingthrough an oxide-filled trench.

FIGS. 22A and 22B show isolation structures and the vertical separationbetween a deep isolating layer and a heavily-doped region at the surfaceof the substrate in each structure.

FIG. 22C is a graph of the breakdown voltage between the deep isolatinglayer and the heavily-doped region in each of the structures shown in.FIGS. 22A and 22B.

DESCRIPTION OF THE INVENTION

FIGS. 6A-6V illustrate a number of basic structures that can be formedusing the methods of this invention. The general objective is to form anumber of implanted wells lying over a deep implanted “subsurface”layer. These are in effect “building blocks” that can be combined invarious ways in manufacturing a usable device. The deep implanted layersdescribed herein are in contrast to conventional “buried layers”, formedat the bottom of an epitaxial layer before and during the growth of theepitaxial layer. Such pre-epitaxial buried layers necessarily exhibitdopant redistribution during the growth of the epitaxial layer.

FIG. 6A shows a deep implanted N isolating layer 131 in a P substrate130. FIG. 6B shows a deep implanted isolating layer 133 which is brokeninto sections 133 a and 133 b. FIG. 6C shows an implanted P well 134above and separated from N isolating layer 131. Without sidewallisolation regions, however, P well 134 is not isolated from P substrate130. FIG. 6D shows P well 134 touching deep N isolating layer 131, andFIG. 6E shows that P well 134 can be implanted in such way that aportion of well 134 is located on the underside of deep N isolatinglayer 131.

FIG. 6F shows an N well 135 above and separated from deep N isolatinglayer 131; FIG. 6G shows N well 135 overlapping deep N isolating layer131; and FIG. 6H shows an annular well 135 that merges with deep Nisolating layer 11, forming an fully isolated region 140 surrounded by Nwell 135 on its sides and by deep N isolating layer 131 on its bottom.

FIG. 6I shows P well 134 abutting N well 135, with N well 135 touchingdeep N isolating layer 131. FIG. 6J is similar to FIG. 6I except that Pwell 134 is spaced from N well 135. FIG. 6K shows a stricture formed bya complementary well process where the entire surface of P substrate 130is occupied by either a P well 134 or an N well 135 and N buried layerunderlies and touches the P and N wells. If N well 135 forms a ring orannular structure around the center section of P well 134, then thiscenter sect-ion will become fully isolated in the same manner as theisolated structure shown in FIG. 6H. FIG. 6L is similar to FIG. 6H butshows a structure in which one of the P wells 134 is implanted to ashallower depth than the N wells 135 and is enclosed in an annular ringformed by N wells 135. FIG. 6M is similar to FIG. 6L but the P well 134extends below the deep N isolating layer 131. In both FIGS. 6L and 6M Pwell 134 becomes fully isolated from P substrate 130.

FIG. 6N shows an implanted P subsurface layer 136 in P substrate 130.While there are no PN junctions in this embodiment, it would have aninverse or “retrograde” doping concentration, i.e., the dopingconcentration of P-type impurity increases in the direction downwardfrom the surface of substrate 130 towards deep P layer 136. FIG. 6Oshows P well 134, which could be fully implanted, merging with deep Player 136. Again, this structure could have a retrograde dopingconcentration.

FIGS. 6P-6R show structures containing deep N isolating layer 131 anddeep P layer 136 together in P substrate 130. Since deep layers 131 and136 have different lateral dimensions, different masks were used informing them. The mask used to form deep layer 131 would have an openingthat is wider that an opening in the mask used to form deep layer 136.In other embodiments, the same mask could be used to form a deep N layerand a deep P layer, in which case the layers would have roughly the samelateral dimension. FIG. 6P shows deep P layer 136 extending both upwardand downward from deep N layer 131. FIG. 6Q shows deep Player 136extending only upward from deep N 131. FIG. 6R shows deep P layer 136extending only downward from deep N layer 131.

The structure shown in FIG. 6Q can be achieved by implanting deep Player 136 at an implant energy such that it has a projected range lessthan that of deep N layer 131. The structure shown in FIG. 6R can beachieved by implanting deep P layer 136 at an energy such that it has aprojected range deeper than deep N layer 131. The structure of FIG. 6Pcan be achieved using two implants to form deep P layer 136, one deeperthan deep N layer 131, the other more shallow than deep N layer 131.Another method of fabricating the structure of FIG. 6P involves a singleimplant of boron to form deep P layer 136, the implant having the samerange as the phosphorus implant used to form deep N layer 131, but alower dose. The exposed portion of deep P layer 136 above and below deepN layer 131 occurs because boron exhibits a larger degree of stragglethan phosphorus at any given depth.

FIG. 6S shows an embodiment that includes P well 134, deep P layer 136and deep N 131, with P well 134 and deep P layer 136 sitting above deepN layer 131. P well 134 and deep P layer 136 would have a retrogradedoping concentration. FIG. 6T is similar to FIG. 6S except that deep Player 136 extends both upward and downward from deep N layer 131,comprising one of two implants. FIG. 6U is also similar to FIG. 6S butshows deep P layer 136 being separated from deep N layer 131. Theportion of P substrate that separates deep P layer 136 and deep N layer131 is formed by differences in implant energies rather than anepitaxial process and therefore the separation distance can be set withgreat precision.

FIG. 6V shows an N well 135, similar to the one shown in FIG. 6L,implanted around P well 134 and deep P layer 136. P well 134 and deep Player 136 are arranged similar to the structure shown in FIG. 6S. ThusFIG. 6V shows that a fully isolated, retrograde P well can be formedvery precisely and with a minimal thermal budget.

In summary, FIGS. 6A-6V show that, without relying on epitaxial growth,a tremendous variety of structures can be fabricated using theprinciples of this invention. Because no epitaxial process is involved,the components of the structure can be formed very precisely and withless lateral movement, less variability and greater control overbreakdown voltages. Moreover, the doping concentrations can be either anormal Gaussian profile extending downward from the surface of substrateor an inverse or retrograde profile (Gaussian extending upward towardsthe surface of the substrate). Combined implants may be used tosynthesize a non-Gaussian profile.

FIGS. 7A-7C illustrate CMOS structures containing some of elements shownin FIGS. 6A-6V. All of the structures are capable of beingmonolithically integrated without the need for growing an epitaxiallayer.

FIG. 7A shows a fully isolated CMOS device 150 fabricated in accordancewith this invention. CMOS device 150 contains a PMOSFET 169 a and anNMOSFET 169 b NMOSFET 169 b is formed in a P well 154 b and is generallysimilar to NMOSFET 30 b shown in FIG. 2A. Included in NMOSFET 169 b arean N+ source region 159 b, an N+ drain region 163 b and a P+ bodycontact region 157 c. N-regions 163 a and 163 b are lightly-doped drainregions. A gate 155 b is formed over a gate oxide layer 156 b. A LOCOSfield oxide layer 160 and a second oxide layer 161 overlie the surfaceof P substrate 151.

P well 154 b overlies deep N layer 152 a and is surrounded by N well 153a, which together isolate NMOSFET 169 b from P substrate 151. In thiscase, however, N well 153 a also contains a PMOSFET 169 a, generallysimilar to PMOSFET 30 a shown in FIG. 2A, which is also isolated from Psubstrate 151. Included in PMOSFET 169 a are a P+ source region 157 a, aP+ drain region 157 b and an N+ body contact region 159 a. P-regions 158a and 158 b are lightly-doped drain regions. A gate 155 a is formed overa gate oxide layer 156 a.

In other embodiments, N well 153 a would not have to contain a PMOSFETbut could be wrapped around P well 154 b thereby simply providing P well154 b with isolation from P substrate 151. The width of the isolatingring represented by well 153 a can be widened to improve the isolatingcapability of the structure.

A diode 169 c is also formed in an N well 153 c. Diode 169 c includes aP+ anode region 157 d and an N+ cathode region 159 e. A deep N layer 152b underlies N well 153 c and suppresses the injection of holes in Psubstrate 151 to prevent PNP bipolar action involving P+ anode region157 d, N well 153 c and P substrate 151. Lateral PNP conduction may befurther suppressed by widening N well 153 c to increase the lateralextent of N well 153 c beyond P+ region 157 d.

Alternatively, if even greater isolation between PMOSFET 169 a, andNMOSFET 169 b were desired, PMOSFET 169 a could be placed in an N wellseparate from N well 153 a, and N well 153 a could be used solely forisolating NMOSFET 169 b from the substrate.

FIG. 7B illustrates an embodiment that contains an NPN transistor 169 dand an N-channel lightly-doped drain lateral double-diffused channel.MOSFET (LDMOS) 169 e.

In NPN transistor 169 d, N+ region 159 g acts as the emitter, P+ region157 e and P well 154 c act as the base and N well 153 d and deep N layer152 e act as the collector. Deep N layer 152 c isolates the base (P well154 c) from P substrate 151.

In N-channel LDMOS 169 e, N+ region 159 i, N well 153 f and deep N layer152 d act as the drain, with N well 153 f serving as the lightly-dopedportion of the drain to spread the voltage drop laterally along thelateral extent of N well 153 f and away from N+ region 159 i and P well154 d. P+ region 157 f and P well 154 d act as the body of the MOSFET,and N+ region 159 i acts as the source. As is customary, the source andbody are shorted together by means of metal source-body contact 162,although the source and body could be biased separately if separatesource and body contacts were employed. The body region (P+ region 157 fand P well 154 d) is isolated from P substrate 151 by N well 153 f anddeep N layer 154 d.

FIG. 7C illustrates three devices: a P-channel LDMOS 169 f, a substratePNP transistor 169 g, and a non-isolated NMOSFET 169 h.

In P-channel LDMOS 169 f, P+ region 157 g and P well 154 e act as thedrain, with P well 154 e serving as the lightly-doped extension of thedrain to help spread the voltage drop laterally between P+ region 157 g,and N well 153 h. The voltage at P+ region 157 g should not exceed thebreakdown voltage of the junction between P well 154 e and deep N layer152 e. N+ region 159 k, N well 153 h, and deep N layer 152 e act as thebody, and P+ region 157 h acts as the source, Again, the source and bodyare typically shorted together by means of metal source-body contact167, as shown, but could be biased separately. The drain (P+ region 157g and P well 154 e) is isolated from P substrate 151 by N well 153 h anddeep N layer 152 e.

Substrate PNP transistor 169 g includes P+ region 157 k which acts asthe emitter N+ region 159 m and N well 153 j which act as the base, andP+ region 157 i and P well 154 f which are tied to P substrate 151 andtogether act as the collector. Substrate PNP transistor 169 g may leadto currents in P substrate 151, so the current density of substrate PNPtransistor 169 g is normally limited to small signal applications.

NMOSFET 169 h is similar to NMOSFET 169 b (FIG. 7A) except that it body(P well 154 f) is not surrounded by an N well and deep N layer and thusis not isolated from the substrate. NMOSFET 169 h includes an N+ sourceregion 159 n, an N+ drain region 159 p, a polysilicon gate 155 e and agate oxide layer 156 e. A P+ region 157 j provides contact to the body(P well 154 f). The decision whether to make the NMOSFET isolated ornon-isolated is a matter of design choice.

FIGS. 8A-8H illustrate a process forming an isolated P well inaccordance with the invention. In FIG. 8A, an oxide layer 170,preferably thick, has been formed on a P substrate 173. A photoresistlayer 171 is deposited over oxide layer 170 and patterned, usingconventional photolithographic techniques, to form an opening. As shownin FIG. 8B, oxide layer 170 is etched through the opening. Either acontrolled etch can be performed, leaving a portion of oxide layer 170in place, or the portion of oxide layer 170 under the opening can becompletely removed and a new thin oxide layer can be grown In eithercase, a thin oxide layer 170 a remains over the P substrate 173 in theopening. An N-type dopant such as phosphorus is implanted through thinoxide layer 170 a to form a deep N layer 174. Oxide layers 170 and 170 aand photoresist layer 171 are then stripped, leaving the structure shownin FIG. 7C, with a compact, highly defined deep N layer 174 floating inP substrate 173.

Table I summarizes the processing steps used in the formation of deep Nlayer 174 and some possible variants of the process.

TABLE I Possible Preferred Element Range Range Target CriteriaImplant-blocking 100 Å-5 μm 1 μm-3 μm 2 μm Oxide 170 plus mask oxide(170) - thickness 171 must block implant Implant-blocking 30 min-10 hrs2-4 hrs 3 hrs @ No thermal limit oxide (170) - @ 900-1200° C. @1000-1100° C. 1050° C. oxidation conditions Pre-implant oxide 100-1000 Å100-300 Å 200 Å To prevent surface (170a) - thickness damage Photoresistblocking 1-5 μm 2-3 μm 2.5 μm Mask 171 plus oxide mask (171) - thickness170 must block implant Deep N phosphorus 100 keV-3 MeV 1.5-2.3 MeV 2.3MeV Implant as deep as implant (174) - energy possible

The conditions described in Table I may be altered, depending on therequired voltages formed in thee layers above deep N layer 174. Ingeneral, the higher the voltage rating of the device, the deeper the Nlayer should be implanted. Deeper implants are also needed in the eventthat any significant high temperature diffusions/oxidations (thermalbudget) occur after the implant of the deep N layer.

Alternatively, oxide layer 170 may be grown thin and left in placeduring the implantation so that an etchback to form layer 170 a is notrequired.

A pad oxide layer 172 is formed on the surface of P substrate 173, and asecond photoresist layer 176 is deposited and patterned, leaving anopening as shown in FIG. 8D, The opening is preferably annular (i.e., asolid pattern with holes formed in it). An N-type dopant such asphosphorus is implanted, creating, an N well 175, which because of theannular shape of the opening, surrounds any and all isolated portions177 of P substrate 173.

Photoresist layer 176 is stripped, and a third photoresist layer 179 isdeposited and patterned to form an opening over N well 175. A P-typedopant such as boron is implanted through the opening to form anisolated P well 178, having a dopant concentration greater that thedopant concentration of P substrate 173. The resulting structure isshown in FIG. 8E. Not all isolated regions 177 must receive the ionimplant used to form P well 178.

The processing conditions that may be used in the formation of N well175 and P well 178 are described in Table II, including some processvariants.

TABLE II Preferred Element Possible Range Range Target CriteriaPre-implant 500-1000 Å 50-200 Å 100 Å Low- oxide temperature to avoiddeep N updiffusion Implant 1-5 μm 2-3 μm 2.5 μm Must block blockingmasks well implants (176, 179) N well 175 1E11-1E14 cm⁻², 1E11-1E12cm⁻², 1E12 cm⁻², N well should (phosphorus) 150 keV-2 MeV E<300 keV 250keV or overlap deep N implant (one or or 1E12-1E14 cm⁻², 3E13 cm⁻²,layer conditions multiple E>700 keV 1 MeV implants) P well 178 1E11-1E14cm⁻² 1E11-1E12 cm⁻², 1E12 cm⁻², 150 keV Projected range (boron) implant100 keV-1.4 MeV E<200 keV or of P well conditions (one or 5E12-1E14cm⁻², 3E13 cm⁻², 500 keV should be or multiple E>400 keV below projectedimplants range of N well

The P well and N well can be made from single implant but then must bedoped heavily to avoid punchthrough breakdown. In Table II the targetexemplifies a two-implant well formation comprising a shallow and adeeper implant This method works well for the fabrication of 5V CMOSdevices and produces acceptable results for the fabrication of 12V CMOSdevices.

The shallow implants set the basic CMOS device characteristic beingsufficiently heavily-doped top prevent channel punchthrough but lightdoped enough to exhibit a threshold voltage close enough to the targetthat a shallow V, adjusting implant is able to set the final thresholdvoltage value (without excessive counterdoping). The well doping mustalso be light enough to meet the required breakdown voltage. A “shallow”implant in this context is an implant at an energy under 200 keV forboron or under 300 keV for phosphorus, and a “deep” implant is animplant at an energy over 400 keV for boron or over 700 keV forphosphorus. The dose of the deeper implants is preferably higher to helpsuppress parasitic bipolar action. The P well, however, must not be asdeep as the deep N layer; otherwise the P well may counterdope the deepN layer and the isolation capability of the device will be degraded.

The well doping profile may also be constructed b additional implantsbut then the surface dose may be further reduced accordingly. Forexample a 12V compatible N well as described may comprise a 1E12 m⁻²phosphorus implant at 250 keV and a 3E13 cm⁻² phosphorus implant at 1MeV. An added implant, for example, an extra 7E12 cm⁻² may be includedat an intermediate energy such as 600 KeV. The lower the energy of theadded implant, the more likely the surface concentration may beaffected.

In a 5V only device the need for multiple chained implants is less thanin 12V devices, since all the implanted layers can be formed closer tothe surface, i.e., at lower implant energies. Since the dopant isconstrained to a thinner layer, the resulting concentration for a givendose is increased. Accordingly, 5V CMOS wells may be produced with alower implant dose but still produce a layer having a higher dopantconcentration.

A 5V N well may comprise a deep implant of only 5E12 cm⁻² at 500 keV,one-half the energy and one-sixth the dose of the deeper 12V deep well.The shallow implant of a 5V N well may comprise a dose of 6E11 cm⁻² at250 keV, not a substantial difference in energy from a 12V device. Thelower dose is not so critical since the PMOS device's characteristic ismore a function of a subsequent V, adjusting implant than the wellitself. Moreover, PMOS devices are less likely to exhibit parasiticsnapback than NMOS devices.

The fabrication of a 5V NMOS in a 5V P well is substantially differentfrom the fabrication of a 12V NMOS in a 12V P well. Both the 5V P welland the 12V P well comprise the combination of a deep implant to preventbulk punchthrough and a shallow implant to prevent surface punchthroughIn both cases the shallow implant has its peak near the surface, aconsequence of a 40 keV implant. The shallow implant of the 5V P wellgenerally has a higher dose than the 12V P well ranging from 20% higherto as much as double, primarily to prevent punchthrough in the shorterchannel length 5V device.

The deep boron implant used in the 5V P well is however, both shallowerand lighter than the 12V P well. For example, the 5V P well may comprisean implant dose of around 1 to 2E13 cm⁻² at an energy of 250 keV. The12V P well in contrast uses a deep implant at an energy near 500 keV andan implant dose of 3E13 cm⁻² to 5E13 cm⁻² (nearly twice the energy andtwice the dose of the 5V P well). While it may seem counter intuitive touse a higher dose implant for a higher voltage device, bulk punchthroughand snapback phenomena occur in higher voltage devices farther away fromthe surface than in low voltage devices. The parasitic bipolarphenomenon is exacerbated in the bulk due to a higher minority carrierlifetime. Impact ionization is also worsened by the alignment of thepath of current through high electric field regions of a the draindepletion region in a saturated MOSFET. Increasing the deep implantdoping minimizes these effects.

As shown in FIG. 8F, a silicon nitride layer 180 is deposited over padoxide layer 173 a. Nitride layer 180 is patterned and etched, usingconventional photolithographic techniques, to expose certain areas ofpad oxide layer 173 a. A photoresist layer 181 is then deposited overnitride layer 180 and patterned to create an opening, over P well 178. AP-type dopant such as boron is implanted through the openings in nitridelayer to form enhanced-concentration P field doped (PFD) regions 182 inP well 178 and in other P wells in the structure.

As shown in FIG. 8G, photoresist layer 181 is removed, and an N-typedopant such as phosphorus or arsenic is implanted thorough the openingsin nitride layer 180 to form enhanced-concentration N field doped (NFD)regions 183. The dopant that goes into N well 175 forms NFD regions 183,while the NFD dopant that goes into P well 178 is not concentratedenough to completely counterdope the PFD regions 182. Unlike the casewith conventional CMOS devices, the thermal oxidation time andtemperature must be held to a minimum to prevent redistribution of thedopant in the deep N layers and in the N wells and the P wells,especially the heavily-doped portions thereof. For field oxidesapproximately 4000 Å in thickness, NFD implant of around 5E13 cm⁻² areemployed while PFD implants twice that dose are required. The implantsare at a low energy, typically about 50 keV.

P substrate 173 is subjected to a low-temperature oxidation, producingfield oxide layers 184 in the portions of P substrate that underlie theopenings in nitride layer 180. This is the well-known local oxidation ofsilicon (LOCOS) process. The anneal also drives in PFD regions 182 andNFD regions 183, thereby forming field dopant regions which togetherfield oxide layers 184 provide a higher field threshold and preventinversion in the areas between the active devices.

Next, a sacrificial oxide layer (not shown) is formed on the surface ofP substrate 173, and a gate oxide layer 185 is grown. The isolatedstructure shown in FIG. 8H is ready for the formation of NFD regions,for example, the CMOS devices shown in FIG. 7A.

FIGS. 9A-9G are schematic diagrams that represent the devices shown inFIGS. 7A-7C, which have been similarly numbered in FIGS. 9A-9G. FIG. 9Ashows PMOSFET 169 a and NMOSFET 169 b (FIG. 7A). NMOSFET 169 b isisolated from P substrate 151 by diode 193, which represents the PNjunction between P well 154 b and deep N layer 152 a, and by diode 197,which represents the PN junction between deep N layer 152 a and Psubstrate 151. Diodes 193 and 197 are back-to-back diodes thatcompletely isolate NMOSFET 169 b from P substrate 151. The cathodes ofdiodes 193 and 197 (i e., the deep N layer) can be biased to anarbitrary potential, labeled as “FI” (an acronym for “floor isolation”)but are typically biased at the most positive potential on the chip.This potential is also commonly used to bias the source of PMOSFET 169a.

FIG. 9B, diode 169 c (FIG. 7A) is isolated from P substrate 151 by adiode 200, which represents the junction between deep N layer 152 b andP substrate 151. In operation, the cathode of (pin K) of diode 169 cmust remain more positive than ground (the anode of diode 200). FIG. 9Cshows NPN transistor 169 d (FIG. 7B), with the diode 202 representingthe junction between P substrate 151 and deep N layer 152 c. FIG. 9Dshows the substrate PNP transistor 169 g (FIG. 7C). It is important thatthe collector (P+ region 157 i) be physically located near the base (Nwell 153 i) so that the current does not flow too far into and along theP substrate 151.

FIG. 9E shows the non-isolated NMOSFET 169 h (FIG. 7C), having astructure similar to NMOS 169 b of FIG. 9A but without the deep N layerforming diodes 193 and 197. FIG. 9F shows lateral high-voltage PMOSFET169 f (FIG. 7C). Diode 212, represents the junction between deep N layer152 e and P substrate 151. The body (N well 153 h) is shorted to thesource (P+ region 157 h), and “anti-parallel” diode 211 represents thejunction between the body and the drain (P well 154 e). FIG. 9G showsthe lateral NMOSFET 169 e (FIG. 7B). Diode 209 represents the junctionbetween deep N layer 152 d and P substrate 151. The body (P well 154 d)is shorted to the source (N+ region 159 j), and “anti-parallel” diode208 represents the junction between the body, and the drain (N well 153f).

FIGS. 10A-10F illustrate how the depth of the deep N layer in thesubstrate can be varied while still providing an isolation structure.

FIG. 10A shows a deep N layer 221 that is implanted to a depth d₁ into aP substrate 221. Deep N layer is implanted through an opening in aphotoresist layer 223 and through an oxide layer 222. In FIG. 10B,photoresist layer 223 has been removed and replaced by a photoresistlayer 224, which is patterned with an annular opening. Dopant isimplanted through the annular opening in photoresist layer 224 to forman N well 225, which merges with deep N layer 221 to form an isolationstructure. Alternatively, the ring can be formed with a separate implanthaving a higher dose that the N well.

In FIG. 10C, a thick oxide layer 232 and a photoresist layer 234 havebeen deposited on a P substrate 230 and patterned to provide an opening.A thin oxide layer 233 is grown in the opening. Alternatively, oxidelayer 232 can be etched back to form the thin oxide layer. A deep Nlayer 221 is implanted in P substrate 230 through the thin oxide layer233. Photoresist layer 234 is removed, and a photoresist layer 235 isdeposited with an annular opening, as shown in FIG. 10D. Deep N layer221 is implanted to a depth d₂ greater than d₁ that makes it difficultto form an isolation structure using a single N well such as N well 225shown in FIG. 10B. Instead, as shown in FIGS. 10D and 10E, first anintermediate medium-depth N (MN) well 236 is formed on the topside ofdeep N layer 231, and this is followed by the implant of a second N well237, which reaches to the surface of P substrate 230 and merges with Nwell 236. Typically the dose of the implant that forms N well 237 wouldbe such as to yield a retrograde doping profile for N wells 236 and 237,i.e., the doping concentration of N well 237 is less than the dopingconcentration of N well 236, which in turn is less than the dopingconcentration deep N layer 231, although N well 236 and deep N well mayalso have the same doping concentration.

The result is an isolated region 238 of P substrate 230. Oxide layers232 and 233 and photoresist layer 235 a-e stripped, producing theisolation structure shown in FIG. 10F, which includes a stack of Nregions that extend upward from deep N layer 231 to the surface of Psubstrate 230. Any number of N regions could be stacked in this way tocreate isolation structures of various depths. The stack of N regionscan be formed very rapidly with pulsed implants of varying energies anddoses to achieve an isolation structure of whatever size and dopingprofile are desired. The top N region N well 237, may be a (CMOS N wellor a dedicated isolation implant. The sidewall consisting of MN 236 andN well 237 may also be formed using a channel implant or multipleimplants at different energies.

The implants shown in FIGS. 8B, 8D and 10A-10E are preferably performedusing a high energy implanter, which may achieve an implant energy of3,000,000 eV of higher, and by limiting the amount of thermalprocessing, following the implants to avoid diffusion of the implanteddopants. The location of the implanted dopants, both vertically andlaterally, can be determined with great precision in sharp contrast tothe uncertainty associated with controlling the results of thermaldiffusion processes. As a result the isolation regions are compact andpredictably located, and the packing density of the transistors or otherdevices in the substrate can be increased.

In the processes and structures defined thus far, the implants wereperformed through oxide layers of uniform thickness (except for areasmasked from ion implantation). The resulting wells and deep layers havedopant profiles and junctions that run essentially parallel to thewafer's original flat surface.

FIGS. 11A-11G show a method of forming an isolation region using astair-step oxide. Step oxides can be used to shape or contour thejunctions. The process starts with the formation of a thick oxide layer241 over a P substrate 240. A photoresist layer 242 is deposited on topof oxide layer 241 and patterned with an opening, through which aportion of oxide layer 241 is etched, as shown in FIG. 11B. A thinneroxide layer 243 is grown in the opening as shown in FIG. 11C. Anotherphotoresist layer 244 is deposited and patterned, as shown in FIG. 11D,this time with a smaller opening. A portion of oxide layer 243 isremoved through the smaller opening, photoresist layer 244 is removed,and a thinner oxide layer 245 is grown in the opening, yielding thestair-step structure shown in FIG. 11E.

An N-type dopant such as phosphorus is implanted at a single energythrough oxide layers 241, 243 and 245. Because of the differentthicknesses of the oxide layers 241, 243 and 245, the range of theimplant varies, producing a deep N layer 246 a and N wells 246 b and 246c, as shown in FIG. 11F. Oxide layer 241 is sufficiently thick that itprevents essentially all of the dopant from reaching P substrate 240.With a short anneal a saucer-shaped isolation structure 247, shown inFIG. 11G, is formed, enclosing an isolated region 248 of P substrate240.

In contrast to the prior structures the depth of the implanted layervaries laterally along, and across the chip, wherever an oxide stepoccurs. The number of steps can be increased to create a more gradual,smooth dopant profile. To create a continuously varying junction, agraded oxide may be used.

FIGS. 12A-12F show a process of forming an isolation structure that usesa LOCOS (local oxidation of silicon) technique to form the graded oxide.The process starts with a P substrate 250, on which a silicon oxidelayer 251 and a silicon nitride layer 252 are deposited, as shown inFIG. 12A. Nitride layer 252 is etched, using conventionalphotolithography, to form openings 253, as shown in FIG. 12B. Thestructure is then subjected to a LOCOS process to grow a thick fieldoxide layer 254, shown in FIG. 12C, including the well-known “bird'sbeak” formations 255 where nitride layer 252 is bent upward by thegrowing oxide layer.

Nitride layer 252 is then removed, as shown in FIG. 12D, leaving anopening 252 a where P substrate is covered only by oxide layer 251. AnN-type dopant such as phosphorus is implanted to form a deep N layer256, shown in FIG. 12E. N layer is buried in the region under opening252 a curves upward to the surface of P substrate 250 in the area underthe bird's beak formations 255. In one embodiment, the dopant does notpenetrate field oxide layer 254. The result is shown in FIG. 12F, withan isolated region 257 of P substrate 250 enclosed by N layer 256.

Numerous variations of this process are possible, several of which areshown in FIGS. 12G-12O. FIG. 12C shows an embodiment in which twoopenings are formed in the field oxide and N layers 256 a and 256 b areformed under the two openings, enclosing two isolated regions 257 a and257 b, respectively. Provided that the segment 254 of the field oxidelayer is sufficiently long, the N layers 256 a and 256 b remainseparate. Additional P-type dopant may also be introduced between thewells. The structure shown in FIG. 12H is similar to that of FIG. 12G,except that an N well 258 and a P well 259 have been formed in theenclosed region above deep N layer 256 a.

In FIG. 12I, N well 258 has been formed in the region above N layer 256a and P well 259 has been form in the region above N layer 256 b. Adielectric layer 260 has been deposited over the entire structure Twocontact openings have been formed in dielectric layer 260 and N-typedopant has been implanted through the contact openings to form N+contact regions 261 a and 261 b. The openings are filled with metal toform contacts 262 a and 262 b. Thus N layer 256 a is electricallycontacted by metal contact 262 a, and N layer 256 b is electricallycontacts by metal contact 256 b, allowing N layers 256 a and 256 b to bebiased at desired potentials. Other contacts can be formed at the sametime to connect to the devices fabricated in the isolated deep N rings.

The structure shown in FIG. 12J is similar, except that N layers 256 aand 256 b are linked by an N layer 264 beneath region 254 of the fieldoxide. This is accomplished by masking the structure with a photoresistlayer 270, as shown in FIG. 12K, and implanting the dopant withsufficient energy that it penetrates the field oxide region 254 but doesnot penetrate the photoresist layer 270.

Alternatively, if it is desired to isolate N well 258 and P well 259,the structure can be masked, and a P-type dopant such as boron can beimplanted to form a P field dopant (PFD) 271 under field oxide region254, as shown in FIG. 12I. FIG. 12M shows the implanting of the P fielddopant through an opening in nitride layer 251 which is patterned usinga photoresist layer 252 b. This occurs prior to the deep N high-energyimplant. FIG. 12M shows essentially the same stage of the process thatis shown in FIG. 12B, with the patterned nitride layer 252 overlying theoxide layer 251. The P dopant is implanted through opening 253 to formPFD 271. After field oxide 254 is grown, PFD 271 remains submerged underfield oxide 254, as shown in FIG. 12N. Then the deep N layer implant canbe performed.

Alternatively, PDF 271 could be formed by implanting dopant at a highenergy through field oxide 254, after the field oxide is formed.

FIG. 12O shows a combination of FIGS. 12K and 12L, with PFD 271isolating N well 258 from P well 259, and N layer 264 linking N layer256 b with an adjacent N layer (not shown).

FIG. 13 provides a summary of several processes that can be used to forma twin well CMOS device. The upper path represents a conventionaldiffused well process using a high thermal budget. The lower pathsportray two variants of a low thermal budget process in accordance withthis invention. In one variant an initial oxide layer is formed and thesurface is masked for the implanting of a deep N layer. After the deep Nlayer has been implanted, the surface is masked for the implanting ofthe sidewalls of the isolation structure. Alternatively, a LOCOS processcan be performed and wraparound isolation structure can be formed with ahigh energy implant (as shown in FIGS. 12A-12F).

After the isolation structure has been formed, complementary N and Pwells can be formed, each after a masking step. With the conventionalprocess and with the floor isolation and sidewall isolation process aLOCOS process is performed to grow the field oxide regions. With thewraparound process, the field oxide regions have already been formed, sothe process is complete after the complementary wells have been formed.

FIGS. 14A-14H illustrate a “hybrid” process which combines theconventional diffusion of N and P wells with the subsequent, implantingof an deep N layer. FIG. 14A shows the formation of an oxide layer 301on a P substrate 300. Oxide layer 301 can have a thickness from 100 Å to1 μm, for example. In FIG. 14B oxide layer 301 has been masked with aphotoresist layer 303 a and a portion of oxide layer 301 has been etchedthrough an opening in photoresist layer 303 a to create a thin oxidelayer 302. Oxide layer 302 can have a thickness from 50 to 1,000 Å,preferably about 200 Å. Phosphorus is implanted at a low energy throughthe opening in photoresist layer 303 a to form an N region 304.Typically the energy of the phosphorus implant is 80 to 160 keV and thedose is 1E12 to 5E13 cm⁻¹. As shown in FIG. 14C, N region 304 isdiffused by a thermal process to form an N well 304. The diffusion maytake place at 900 to 120° C. but preferably at around 1050 to 110° C.,with the diffusion time ranging from 4 to 12 hours to reach a junctiondepth of 1 to 2 μm.

A second photoresist layer 303 b is deposited and patterned and anotherportion of oxide layer 301 is etched through an opening in photo-resistlayer 303 b to form a thin oxide layer 306, again about 200 Å thick asshown in FIG. 14D. P-type dopant (boron) is implanted through theopening in photoresist layer 303 b to form a P region 305. As shown inFIG. 14F, P region 305 is diffused by a thermal process to form a P well305. The conditions for implanting and diffusing P well 305 are similarto those described above for implanting and diffusing N well 304. Asindicated P substrate 300 would typically contain a number of N wells304 and P wells 305.

Thus far the process is a conventional high thermal budget process, andthe dopant profiles in N wells 304 and P wells 305 are Gaussian, withthe doping concentration increasing as one moves downward from thesurface of the substrate.

Next, as shown in FIG. 14G, oxide layers 302, 303 and 306 are stripped,and a third photoresist layer 307 is deposited and patterned with anopening over N wells 304 and P wells 305. Using a high-energy implant, adeep N layer 307 is formed in substrate 300. The energy of the implantis set such that deep N layer 307 overlaps and extends below wells 304and optionally, P wells 305. The implant energy ranges from 1.0 to 1.5MeV, with 2.3 being the maximum for high volume, low cost production.Beyond about 2.3 MeV, the commonly available implanters suffer from lowbeam current and long processing times. Photoresist layer 307 is removedyielding the structure shown in FIG. 14H.

FIG. 15A is a graph showing the projected range R_(P)) of boron andphosphorus implants in silicon as a function of implant energy. Curve310 shows the range for “channeling” boron and curve 312 shows the rangefor phosphorus and non-channeling boron. Because the channeling boronmoves through channels in the crystal lattice its range is slightlygreater than the range of the non-channeling boron.

FIG. 15B is a graph of the straggle (ΔR_(P)) for similar implants ofboron and phosphorus. Curve 314 is the straggle for boron and curve 316is the straggle for phosphorus.

FIG. 16A shows the vertical dimension X_(DP)(max) between the bottom ofa P+ region 355 and a deep N layer 354 in a P well 353 and the verticaldimension X_(DP) between the bottom of a P+ region 356 and deep N layer354 in a region 352 of P substrate 351. It is assumed that P well 353 ismore heavily doped than region 352. A diode 352 a, formed by deep Nlayer 354, region 352 and P+ region 356, is essentially a PIN diode,whereas a diode 353 a, formed by deep N layer 354 and P well 354 and Pwell 353, is a PN diode.

FIG. 16B is a graph showing how the breakdown voltages BV of diodes 352a and 353 a, respectively, vary with X_(DP). As would be expected with aPIN diode, the BV of diode 352 a varies is a function of X_(DP) (i.e.,the P substrate region 353 between deep N layer 354 and P+ region 356 isthe intrinsic region of the PIN diode). The BV of diode 353 a isessentially constant until X_(DP) is reduced to a distance (X_(DP))₁ andthen coincides with the BV of diode 352 a at distances less than(X_(DP)). The BV of diode 352 a is higher at values of X_(DP) greaterthan (X_(DP)). FIG. 16C shows the breakdown potential as a function ofthe implant energy of the deep N layer.

FIGS. 16A-16C thus illustrate how one variable, the depth of the deep Nlayer, X_(DP), must be controlled to produce a device having a desiredbreakdown voltage. FIGS. 17A-17E illustrate how another variable, therange of the implant used to form the sidewall of the isolation regionmust be controlled. As shown in FIG. 17A, device 380 contains an deep Nlayer 383 and a sidewall implant 384, which merge in a region designated385 deep N layer 383 and side wall implant 384 form a portion of anisolation region that encloses a region 382 of P substrate 381.

FIG. 17B, is a graph of the dopant profile taken at a section A-A′ ofFIG. 17A. Sidewall 384 has a range R_(P2) with a peak concentration 387,and deep N layer has a range R_(P1) with a peak dopant concentration388. In the overlap region 385, the profiles of deep N layer 383 andsidewall 384 are superimposed, and the dopant concentration fallsgradually in a curve 386 from the peak 387 to the peak 388. At thebottom of deep N layer 383 the net dopant concentration falls to zero atthe junction between deep N layer 383) and P substrate 381. The dopingconcentration in the region of curve 386 should be as high as possibleto achieve good isolation.

FIGS. 17C and 17D show two other possibilities. In FIG. 17C, therespective ranges of sidewall 384 and N buried region 383 are morewidely separated, and as a result the dopant concentration representedby curve 386 falls to a minimum that is below the peak concentration 388of deep N layer 383. This is a less desirable profile than the one shownin FIG. 17B. And FIG. 17D shows an embodiment wherein deep N layer 383and sidewall 384 are separated by an intrinsic P region (as shown as across-section in FIG. 17E). This is an even less desirable embodiment asthe isolation region is very leaky and the electrical behavior of thedevice is unpredictable.

FIGS. 18A-18D illustrate a solution to the problem defined in FIGS. 17Dand 17E, where the deep N layer is so deep that a gap is left between itand the sidewall. In FIG. 18A, a device 400 contains two overlapping,implants 404 and 405 that have been made at different energies anddepths to form a sidewall 406. The lower implant 404 also overlaps withthe deep N layer 403. Together sidewall 406 and deep N layer 403 enclosea region 402 of a P substrate 401.

In FIG. 18B, four implants 411, 412, 413 and 414 have been made atsuccessively greater energies and depths. Each of the implants 411-414overlaps with the overlying and/or underlying implant to form acontinuous vertical sidewall 419. The regions of overlap are designated415-418.

Similarly, depending on the height of the required sidewall any numberof implants can be used. Typically, each implant lasts only a fractionof a second and thus the entire wall can be formed quickly with a rapidsuccession of pulsed implants. FIGS. 18C and 18D are graphs of dopantprofiles taken at vertical cross-sections through a sidewalls formed bya succession of pulsed implants. In both cases the implants NI₁, NI₂ andNI₃ (or deep N layer DN) have projected ranges of RP₁, RP₂ and RP₃ andpeak dopant concentrations of 420, 421 and 422, respectively. In FIG.18D), the dose of each implant is the same and as a result the peakconcentration falls as the implant becomes deeper. This occurs becausethe straggle (ΔRP) increases as the range increases, thus if the dose isthe same the same number of impurity atoms are spread over a greatervertical distance and the peak doping concentration must necessarilybecome lower. This effect is overcome in the embodiment of FIG. 18C byincrease the dose as the implant becomes deeper. As a result, the peakdopant concentration remains about the same in each implant.

FIGS. 19A-19D illustrate the steps of a process for fabricating anisolation region having a sidewall of the kind shown in FIGS. 18A-18D.FIG. 19A shows the implanting of a deep N layer 454 in a P substrate 451through an opening 450 in a photoresist layer 453. Photoresist layer 453is removed and replaced by a photoresist layer 460. As shown in FIG.19B, an opening 462 is formed in photoresist layer 460 and an implant461 is made at an energy somewhat less than the energy used for deep Nlayer 454. This is followed by an implant 463 (FIG. 19C) and an implant464 (FIG. 19D), each of which is made at a successively lower energythrough the same opening 462 in photoresist layer 460. Since thisprocess is carried out at a low temperature, there is very littlehorizontal spreading of the implants 461, 463 and 464, yielding asharply defined, vertical sidewall. The result is an isolation structurethat encloses a region 452 of P substrate 451.

FIGS. 20A-20D show the corresponding steps of a similar processperformed after field oxide regions 481 a and 481 b have been grown onthe surface of P substrate 482 When deep N layer 484 is implantedthrough an opening in photoresist layer 485, field oxide regions 481 aand 481 b cause raised portions 484 a, and 484 b to form in deepisolating layer 484. However, field oxide regions 481 a and 481 b causeimplant 486 to have a saucer-shaped contour which compensates for theraised portions 484 a and 484 b of deep N layer 484 (FIG. 20B).Similarly, implants 488 and 489 also have a saucer shape thatcompensates for the shape of the underlying implant (FIGS. 20C and 20D).As a result, the sidewall shown in FIG. 20D which encloses a region 483of P substrate 482, has essentially the same compact, vertical profileas the sidewall shown in FIG. 19D.

The number of implants can, in effect be increased to infinity byproviding an implant with a continually increasing energy instead ofpulsed implants. If the concentration is to remain the same throughoutthe sidewall, the dose can also be increased with the energy.

Even though, as described above, a sidewall formed by this process has avery compact, vertical shape, there is some unavoidable horizontaldiffusion of the dopant. This is shown in FIG. 21A, where despite anopening 507 in a photoresist layer 506 having a horizontal dimensionY_(PR), the implants 504 and 505 have diffused laterally to dimensionsY_(N11) and Y_(N12), respectively, both of which are slightly greaterthan Y_(PR). In fact the deeper the implant, the greater the extent ofhorizontal diffusion or “straggle”, i.e. Y_(N12) would typically begreater than Y_(N11). Thus, if it is necessary to form a very deepisolation region, the amount of horizontal straggle that inherentlyresults from the deep implants may, exceed what is acceptable to achievethe desired minimum feature size of the device.

One solution to this problem is illustrated in FIG. 21B, where anoxide-filled trench 514 is formed in a P substrate 511. Oxide-filledtrench 514 abuts an deep N layer 513 to form an isolation region thatencloses a region 512 of P substrate 511. This structure could be formedby implanting deep N layer 513, etching the trench, depositing an oxidein the trench (e.g., by a CVD process), and planarizing the top surfaceof the oxide fill.

In some situations it may be difficult to achieve the proper overlapbetween the oxide-filled trench and the deep buried layer. This problemcan be overcome using the technique illustrated in FIG. 21C, where anN-type dopant such as phosphorus is implanted through an oxide-filledtrench 524, i.e., alter the trench has been filled with the dielectric.The surface of P substrate 521 is masked with a photoresist layer 525.Because the oxide in trench 524 is slightly more resistant to thepassage of the dopant than the substrate, a deep N layer 523 having aslight cup or saucer shape is formed, extending downward from the bottomof trench 524 and turning, in a horizontal direction and then turningupwards towards the bottom of a neighboring trench (not shown).Photoresist layer 525 is removed, yielding the structure shown in FIG.21D. Note that for clarity the curvature of deep N 523 is exaggerated.

Another criterion that the designer must be concerned about is thepossibility of punchthrough breakdown between a deep layer and aheavily-doped region at the surface of the substrate. This problem isillustrated in FIGS. 22A and 22B. FIG. 22A shows a device 530 with aregion 532 of P substrate 531 enclosed by a deep N layer 533 andsidewall implants 534 and 535. Deep N layer 533 is separated by avertical distance X_(NIN) from an a N+ region 536 at the surface of Psubstrate 531. Contrast this with device 540 shown in FIG. 22B, which isthe same except that a more heavily-doped P well 537 has been formed inthe enclosed region, and deep N layer 533 is separated from N+ region536 by a vertical distance X_(NPN).

FIG. 22C is a graph showing the variation of the breakdown voltagebetween N+ region 536 and deep N layer 533 as a friction of the implantenergy used to form deep N layer 536 (which is directly related to thevertical distances X_(NIN) and N_(NPN) shown in FIGS. 22A and 22B). Asindicated in device 540 (curve 542) the breakdown voltage remainsessentially constant until the deep N layer becomes quite shallow, wherepunchthrough occurs at V_(PT) (NPN). In device 530 (curve 544) thebreakdown voltage varies directly with the implant energy of the deep Nlayer until punchthrough occurs at V_(PT) (NIN), which is considerablyhigher than V_(PT) (NPN). Thus providing a P well reduces the breakdownvoltage generally but makes the breakdown voltage relatively insensitiveto the vertical distance X_(NPN) until punchthrough occurs. Leaving theP substrate “as is” in the enclosed region increases the breakdownvoltage when the vertical distance X_(NIN) is relatively large, but thebreakdown voltage is sensitive to X_(NIN) and punchthrough occurs at alarger value of X_(NIN).

Processes that rely on high temperature diffusions result in thediffusion and redistribution of all dopants present in the siliconduring the high temperature processes. The total “hot time”, i.e. thetime during which the substrate is subjected to high temperature, iscommonly referred to as the “thermal budget” of a process. Since IC andtransistor fabrication processes generally use a sequence of steps thatmay involve different temperature diffusions of various durations, it isgenerally not easy to compare the cumulative thermal budget of widelydissimilar at processes using only temperature and time. The firstdopants introduced into the silicon in any process however, do in factexperience diffusion during the entire thermal budget of the processes,and therefore the “thermal budget” of a process is measured from thetime that the first dopants are introduced in to the substrate. Themovement of these dopants during thermal processing is governed byFick's law of diffusion as described in A. S. Grove, Physics andTechnology of Semiconductor Devices (1967), p. 50, as an equationdescribing a Gaussian dopant profile of concentration N(x) as a functionof time, diffusivity, and implant dose Q, as given by the equation

${N(x)} = {{No} \cdot {\mathbb{e}}^{\frac{x^{2}}{4{Dt}}}}$where D is the diffusivity of the dopant in the substrate, t is time,and No is the surface concentration at any given time in the diffusionexpressed in terms of the implant dose Q by the relation

${No} = \frac{Q}{\sqrt{\pi({Dt})}}$

The two equations together reveal that an increase in the thermal budgetDt lowers both the surface concentration No and the concentration of thedopant at any depth N(x) in proportion. Rearranging the equation forjunction depth xj of any diffusion yields

$x_{j} = \sqrt{{- 4}({Dt})\mspace{11mu}\ln\mspace{11mu}\left( \frac{N\left( x_{j} \right)}{No} \right)}$where N(xj) is the concentration of the background doping of theopposite conductivity type layer into which the diffusion occurs. So thedepth of a junction is roughly proportional to the square root of its“Dt” thermal budget. Dt can therefore be used to describe a singlediffusion of a sequence of many diffusions of differing time andtemperature simply by summing the Dt value for each portion into a totalDt for the entire process.

The diffusivity D is a function of temperature T, the dopant species(e.g. boron B, phosphorus P, arsenic As or antimony Sb) and in somecases like phosphorus depends slightly on concentration. The diffusivityof these dopants is given in O. D. Trapp et al., SemiconductorTechnology Handbook, (1980 Ed.), p. 4-6, or by simulation.

A process in accordance with this invention may use a very low thermalbudget process such as the one shown in Table I below, for example,wherein the majority of the diffusion, i.e. the largest Dt, occursduring the formation of the gate oxide and the S/D implant oxidation.The motivation for higher temperature gate oxidation (850° C.) is toobtain high quality oxide. The S/D implant oxidation is used to densifythe sidewall oxide of the gate's sidewall spacers, which originally isdeposited.

TABLE III Example of Low Thermal Process Process Temp Time BoronPhosphorus Step (C.) (min) Dt (step) Dt (sum) Dt (step) Dt (sum) Gate Ox1 850 75 0.000340 0.000340 0.00451 0.00451 Gate Ox 2 850 52 0.000236 0000576 0.00313 0.00764 S/D Ox 850 56 0.000254 0.000830 0.00337 0.01101RTA 960 0.4 0.000017 0.000847 0.00005 0.01106 RTA 900 0.3 0.0000030.000850 0.00004 0.01110

Thus the cumulative thermal budget is the sum of all the Dt values ofall the individual steps. In the exemplary process described above, thetotal Dt for boron is 0.00085 μm² and for phosphorus is 0.01110 μm². Ingeneral, a low thermal budget can be considered as one where themajority of its thermal budget occurs in less than 4 total hours at 850°C., or (considering a variety of process flows) where the total Dtthermal budget is under 0.03 μm² for boron or 0.05 μm² for phosphorus.

An alternative embodiment uses a medium thermal budget for a fieldoxidation, or partial well diffusions, that may comprise two to threehours of hot time, at temperatures of 1000° C. or higher, but not above1100° C. (see Table II). During, this period substantial but notintolerable dopant redistribution of dopant may occur, especially indeep implanted layers. Medium thermal budgets can be approximated bythose with Dt values under 0.3 μm² for boron and under 0.5 μm² forphosphorus, or roughly one order of magnitude higher than a low thermalbudget process flow.

TABLE IV Medium Thermal Budget Steps Process Boron Phosphorus Step Temp(C.) Time (min) Dt (step) Dt (step) Field 1000 120 0.0141 0.0212Oxidation 1050 120 0.0481 0.0707 1100 120 0.1458 0.2380

In contrast, conventional high thermal budget processes used for deephigh voltage wells, deep isolation junctions, high voltage bipolar basediffusions and DMOS transistor body diffusions as exemplified in TableIII may comprise very long diffusions, typically from 3 hours to 15hours depending on the required depths. These diffusions causesignificant redistribution of all dopants, especially deep buried layersor junctions.

TABLE V High Thermal Budget Process Steps Process Boron Phosphorus StepTemp (C.) Time (hrs) Dt (step) Dt (step) Conventional Base 1100 6 0.43740.714 Diffusion DMOS Body 1100 10 0.729 1.190 Diffusion JunctionIsolation 1100 15 1.094 1.785 Diffusion

The foregoing embodiments are to be treated as illustrative and notlimiting. Many additional embodiments in accordance with the broadprinciples of this invention will be apparent to person skilled in theart.

1. A process of fabricating a semiconductor device comprising: providinga semiconductor substrate of a first conductivity type, the substratenot containing an epitaxial layer; forming a first mask on a surface ofthe substrate, said first mask having a first opening, said firstopening having a first lateral dimension; implanting a dopant of asecond conductivity type through the first opening to form a first deeplayer; forming a second mask on the surface of the substrate, saidsecond mask having a second opening, said second opening having a secondlateral dimension, said second lateral dimension being entirely enclosedwithin said first lateral dimension; and implanting dopant of the firstconductivity type through the second opening to form a second deeplayer, said second deep layer being laterally constrained within saidfirst deep layer, said second deep layer overlapping said first deeplayer and having an upper boundary above said first deep layer; andimplanting additional dopant of the first conductivity type through thesecond opening to form a third deep layer, said third deep layer beinglaterally constrained within said first deep layer, said third deeplayer overlapping said first deep layer and having an lower boundarybelow said first deep layer.